Report or refuse each unusable file webhooker reads (closes #290)

Audit of the files webhooker reads configuration or required state
from. A missing or zero-length database is reported with the "created
a new, empty database" warning and its path: webhooker.db at start,
and a per-webhook database at the latest at the next start, since
restart recovery now opens the database of every webhook that still
exists. The main database's open errors name webhooker.db
(#459). webhooker resetpw
refuses a zero-length webhooker.db as it refuses a missing one. A
directory in place of a database file or its -wal or -shm is refused,
naming it; beside a -shm directory SQLite opened the database
read-only without a word. The README says how each case is treated.

Model: opus-5-5

internal/resetpw/resetpw.go

@@ -308,7 +308,7 @@ func checkDataDir(dir string) error {
 
 	dbPath := filepath.Join(dir, database.MainDBFileName)
 
-	_, err = os.Stat(dbPath)
+	dbInfo, err := os.Stat(dbPath)
 
 	switch {
 	case errors.Is(err, fs.ErrNotExist):
@@ -319,6 +319,15 @@ func checkDataDir(dir string) error {
 		)
 	case err != nil:
 		return fmt.Errorf("checking %s: %w", dbPath, err)
+	case dbInfo.Size() == 0:
+		// SQLite opens a zero-length file as an empty database, so
+		// it holds no deployment either, and opening it would write
+		// an empty schema into it.
+		return fmt.Errorf(
+			"%w: %s is zero-length. The admin account is created by "+
+				"the first server start",
+			ErrNoDatabase, dbPath,
+		)
 	}
 
 	return nil